The invention relates to a method of tracking position and velocity of objects' borders in two or three dimensional echographic images.
The automatic detection of borders and their rate of displacement is a fundamental topic in image analysis. The ability to automatically detect borders and their velocity allows easy recognition of objects, and an improved understanding of their function. In medical imaging the automatic recognition of an organ would eventually facilitate the extraction of objective measurements and automate some diagnostic processes.
A typical example is given by echocardiography in the imaging of the left ventricle. The possibility of an automatic detection of the endocardial border would give objective measurement of the ventricular volume. In particular, the extreme values (at proto-systole and tele-diastole) are commonly used for clinical diagnosis and the relative diagnostic index is calculated. In addition, the possibility to visualize the border velocities allows an easier assessment of the regional dynamical properties.
In general, and certainly in echographic imaging, the development of reliable methods for the automatic border detection is an extremely difficult task that has not received a generally reliable solution. In fact, in clinical practice, borders are drawn manually by the operator over the physiologically relevant frames of a sequence of images.
A strain rate analysis method in ultrasonic diagnostic imaging is disclosed in WO 02/45587. According to this document strain rate analysis is performed for ultrasonic images in which the spatial gradient of velocity is calculated in the direction of tissue motion. Strain rate is calculated for cardiac ultrasound images in the direction of motion which, for myocardial images, may be either in the plane of the myocardium or across the myocardium. Strain rate information is calculated for a sequence of images of a heart cycle and displayed for an automatically drawn border such as the endocardial border over the full heart cycle. The spatial gradient of velocity used for determining the strain and the displacements of the borders from one frame to a successive frame in a sequence of frames uses Doppler Tissue Imaging, or DTI. This technique allows the measurement of tissue velocity over all points in the ventricular wall. The measurement of velocity itself provides direct information about the wall motion and helps to uncover abnormalities not immediately observable from the visualization in B-mode imaging. The velocity measurement contains information about rigid body displacement, shear, and contraction/distension, the latter being immediately related to the myocardial activity. Post processing of the DTI velocity data allows the evaluation of additional quantities, namely strain-rate and strain, that are strictly related to the regional function. Segmental strain gives a direct evaluation of the degree of contractility of the myocardium during systole, as well as of its relaxation during ventricular filling.
Nevertheless, DTI suffers from a few drawbacks consisting in limitations of the technique. The evaluation of velocity, and to a greater degree, rate and strain, requires a higher frame rate with respect to B-mode imaging because velocity is a more rapidly varying function than B-mode displacement. A Doppler signal requires additional processing with respect to a simple echo.
Doppler tissue imaging suffers further of an intrinsic limitation due to the fact that only the component of velocity along a scanline can be measured. This limitation has several drawbacks. When tissue moves in a direction that is not aligned with the scanline, the Doppler velocity does not reflect the effective tissue kinematics. Only the component of strain and strain rate along the scanline can be evaluated correctly, giving a reduced view of the local deformation state. This limits the application of DTI to the anatomic sites that can be imagined aligned along a scanline. In echocardiography this corresponds essentially to the interventricular septum and to the lateral walls in apical view.